Tag Info

While many iOS and Android apps are not created directly with HTML/CSS and a lot of these resources deal with HTML/CSS, the general principles can and should be implemented in whatever systems the application is being developed in.
A resource you should look at if you're interested in learning a lot about style guidelines is StyleGuides.io which has all ...

In addition to Jons answer, I found that Googles Material Design Guide is so far the most comprehensive as well as beautifully designed Software Design Guide I have come across. They have simply thought of everything. Concept, Structure, Measurements, Relations, Colors. Whatever your UI-heart desires. Still, I should not call it "best" as i your question, as ...

The best way is to hand over working HTML, CSS and JS. That takes a UX team that has those skills at hand, so that's not always viable.
Short of that, the next best way is to ask your developers exactly what they need and want.

These, in Software Engineering terms, are called User Interface Style Guides. They form part of the External Interface Requirements for the Software Requirements Specifications (Wikipedia link)
But this isn't hugely helpful for the person left in charge of actually defining the style of the software.
There is a useful page on the topic at Konigi. This ...